1. Field of the Invention
The present invention relates to a rotary gas separator and, more particularly, to such a rotary gas separator that is adapted to be operated with a progressive cavity pump.
2. Description of Related Art
Electric submergible pumping systems (SPS's) and sucker rod pumps are commonly used to move oil and gas from a subterranean formation through a wellbore to the earth's surface. When viscous fluids and/or fluids with relatively high concentrations of sand are to be moved and, if gas is present, these types of pumps have not proven to be as reliable as desired.
If the ratio of gas-to-oil in fluids being moved is excessive, the gas will come out of solution within the pump and cause gas lock, whereby the pump will cease to move any fluid. To prevent such detrimental gas lock, rotary gas separators are often used to separate as much gas as possible from the wellbore fluids prior to the fluids entering the pump's inlet. Rotary gas separators typically used with SPS's are disclosed in U.S. Pat. Nos. 2,525,233; 3,887,342; 4,481,020; 4,676,308; and 4,981,175. When viscous fluids and/or fluids with relatively high concentrations of sand are to be moved, Moineau pumps or progressive cavity pumps (PCP's) have been operated by a rotating rod string extending to the earth's surface or through a gear reduction system to an SPS's electric motor. Examples of progressive cavity pumps are disclosed in U.S. Pat. Nos. 3,627,453; 4,080,115; and 5,048,622.
The hindrance to the successful operation of a PCP with a drive train of the above types is that the progressive cavity pump inherently causes oscillations and gyrations that propagate through the drive train and destroys the bearings therein. These oscillations and gyrations are hereinafter referred to as "non-coaxial" forces, and have been found to be of sufficient magnitude to damage bearings. The failed pump and drive train must then be removed from the wellbore, which causes the operator to suffer loss of production, loss of revenue and additional repair costs.
The non-coaxial forces are caused by the rotor in the progressive cavity pump being in the shape of a helix contained within a cavity or a flexible lining within a housing, referred to as the stator. The rotor rolls with respect to the stator so that the rotor and stator form a series of sealed cavities which are approximately 180 degrees apart. As one cavity increases in size, its counterpart cavity decreases in size, at exactly the same rate. The driving motion of the rotor is quite complex in that it is simultaneously rotating and moving transversely with respect to the stationary stator's liner. The rotation of the true center of the rotor traces a circle progressing the opposite direction to the rotation of the rotor, but with the same speed. Thus, the rotor driving motion is simultaneously a rotation, an oscillation, and a reverse orbit. It is a combination of these motions acting at the point of interconnection of the drive train that cause the undesired non-coaxial forces to be destructively transmitted to the bearings within the drive train.